Statistics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm33c..03v&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SM33C-03
Statistics
2723 Magnetic Reconnection (7526, 7835), 2744 Magnetotail, 2756 Planetary Magnetospheres (5443, 5737, 6033), 6220 Jupiter
Scientific paper
Some of the auroral polar bright spots at Jupiter are thought to originate through reconnection in the Jovian magnetotail [Grodent et al., 2004; Radioti et al., 2008]. Understanding the statistics of magnetotail reconnection will, therefore, contribute to interpreting the polar aurora. Dynamics in the Jovian magnetotail have been identified from energetic particle data, but a thorough survey of the magnetic field data is yet to be completed. An initial study [Vogt et al., 2007] identified events whose magnetic signatures suggest that magnetic reconnection is taking place in the Jovian magnetotail at distances from 30 to 150 RJ. In the initial analysis, events were characterized by increases in |Bθ|, the north-south component of the magnetic field, over background levels, with less restrictive criteria placed on periods of negative Bθ. We have recently focused on improving our quantitative event selection criteria, with particular emphasis on establishing the required increase in |Bθ|. This has proved to be a delicate task. Although the number and duration of the events vary with the identification criteria, the strongest events are selected by all those applied. Here we analyze events that satisfy quite stringent identification criteria. We distinguish between locations inside or outside of a neutral line, which we infer through the sign of Bθ. The background Jovian field points southward (Bθ> 0) near the equator and we assume that a northward or negative Bθ occurs when the spacecraft is located tailward of a reconnection x-line. Most events with (Bθ < 0) are observed in the post-midnight sector and at radial distances larger than ~90 RJ. Using the sign of Bθ as a proxy for the flow direction, we compare with previous studies that identified intermittent flows from particle anisotropy [Kronberg et al. 2005, 2007]. In the specific cases illustrated there we find that our events occur in conjunction with increases in the particle anisotropies, though our strict selection criteria miss some intervals of increased anisotropy. However, all but one of the disturbed intervals reported by Kronberg et al. [2005, 2007] occur in the post- midnight sector, but we have identified scores of additional events pre-midnight. We have also examined changes to the bendback angle, which represents how swept back the field line is with respect to the radial direction. By assuming that in events lasting less than 5 hours the azimuthal component of the flow changes to conserve angular momentum and that the field is frozen into the flow, we can infer changes of the radial plasma flow from changes of the bendback angle. We find that large, fast changes of the bendback angle occur in the majority of our events. By conservation of angular momentum we expect that the bendback of the field line will increase as plasma flows outward; we also expect negative Bθ during such times. Therefore we examine the correlation between the sign of Bθ and the change of the bendback angle in our events. We will discuss the distribution of events and their inferred properties as functions of radial distance and local time.
Galland Kivelson Margaret
Joy Steven P.
Khurana Krishan K.
Vogt Marissa F.
Walker Ray J.
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